Touch panels have recently become widely adopted as the input device for high-end portable electronic products such as smart-phones and tablet PCs. Although, a number of different technologies can be used to create these touch panels, capacitive systems have proven to be the most popular due to their accuracy, durability and ability to detect touch input events with little or no activation force.
The most basic method of capacitive sensing for touch panels is demonstrated in surface capacitive type touch panels (also known as self-capacitance type touch panels), for example as disclosed in U.S. Pat. No. 4,293,734 (Pepper, Oct. 6, 1981). A typical implementation of a surface (self) capacitance type touch panel is illustrated in FIG. 1 and comprises a transparent substrate 10, the surface of which is coated with a conductive material that forms a sensing electrode 11. One or more voltage sources 12 are connected to the sensing electrode, for example at each corner, and are used to generate an electric field which extends above the substrate. When a conducting object, such as a human finger 13, comes into close proximity to the sensing electrode, a capacitor 14 is dynamically formed between the sensing electrode 11 and the finger 13 and this field is disturbed. The capacitor 14 causes a change in the amount of current drawn from the voltage sources 12 wherein the magnitude of current change is related to the distance between the finger location and the point at which the voltage source is connected to the sensing electrode. Current sensors 15 are provided to measure the current drawn from each voltage source 12 and the location of the touch input event is calculated by comparing the magnitude of the current measured at each source. Although simple in construction and operation, surface capacitive type touch panels are unable to detect multiple simultaneous touch input events as occurs when, for example, two or more fingers are in contact with the touch panel.
Another well-known method of capacitive sensing applied to touch panels can be found in projected capacitive type touch panels (also known as mutual capacitance type touch panels). In this method, as shown in FIG. 2, a drive electrode 20 and sense electrode 21 are formed on a transparent substrate (not shown). The drive electrode 20 is fed with a changing voltage or voltage excitation signal by a voltage source 22. A signal is then generated on the adjacent sense electrode 21 by means of capacitive coupling via the mutual coupling capacitor 23 formed between the drive electrode 20 and sense electrode 21. When a conductive object such as a finger 13 is brought into the proximity of the electrodes, the magnitude of the mutual capacitance 23 is altered according to the distance between the conducting object and the electrodes. A current measurement means 24 is connected to the sense electrode 21 and provides a measurement of the size of the mutual coupling capacitor 23. A touch input event may therefore be detected by monitoring the output of the current measurement means 24. As is well-known, by arranging a plurality of drive and sense electrodes in an array, such as a two-dimensional matrix array, this projected capacitance sensing method may be used to form a touch panel device. An advantage of the projected (mutual) capacitance sensing method over the surface (self) capacitance method is that multiple simultaneous touch input events may be detected. The projected (mutual) capacitance sensing method is also suitable for detecting the proximity of non-conductive objects. In this case, the permittivity of the non-conductive object, if different from the permittivity of the air, results in a change in the magnitude of the mutual capacitance 23.
Although projected capacitive type touch panel devices such as those described above have been widely adopted in consumer electronic products, it is desirable to further improve their performance by addressing the current limitations of this sensing method. In particular, the accuracy at which the location of objects touching the touch panel surface can be determined and the minimum size of touching object are limited by the relatively low signal-to-noise ratio (SNR) of the mutual capacitance measurements. One known approach to increasing the SNR is to optimize the sensitivity of the touch panel to the proximity of an object, such as a finger or stylus, through the design of the touch panel electrodes. For example, U.S. Pat. No. 5,543,588 (Bisset et al, Aug. 6, 1996) discloses a touch panel comprising drive and sense electrodes patterned into diamond shapes. Alternatively, US Patent Application No. 2010/0302201 (Ritter et al, Dec. 2, 2010) discloses a touch panel comprising inter-digitated drive and sense electrodes which may be formed in a single physical layer. A disadvantage of increasing the sensitivity of the touch panel to the proximity of a conductive object in this way however is that the sensitivity of the touch panel to sources of electronic noise and interference may also be increased and the improvement in SNR that may be achieved by this approach is therefore limited.
Sources of noise contributing to the SNR include environmental effects such as changing temperature, humidity and condensation as well as electromagnetic interference emanating from the display device beneath the touch panel and from objects surrounding the device, including the touching object(s) itself. Noise from such sources may be manifested in the touch panel capacitance measurement circuits as correlated or predicable fluctuations in the measured signal. A second approach to increase the SNR is therefore to reduce the effects of these noise sources on the measurement. Typically, the display device is a significant source of interference and one well-known method to reduce the effect of this interference is to synchronize the timing of the display and touch panel functions such that the touch panel is only active to detect touch input when the display function is inactive, for example during the display horizontal or vertical blanking periods. However, such a method does not improve immunity to humidity, condensation or other environmental noise sources and may impose undesirable constraints on the operation of the touch panel and/or the display device which may limit the increase in SNR achievable.
Alternatively, US Patent Application 2009/0135157 (Harley, Nov. 27, 2007) and US Patent Application 2009/0194344 (Harley, Jan. 31, 2008) describe a mutual capacitance sensing device with an additional guard electrode to reduce the sensitivity of the system to humidity and condensation. As shown in FIG. 3, the guard electrode 30 is located between a drive electrode 31 and a sense electrode 32 and is connected to a fixed potential, such as the ground potential. A mutual coupling capacitance 35 that varies in the presence of a touching object, such as a finger 13, is formed between the drive electrode 31 and the sense electrode 32 and a guard capacitance 34 is formed between the guard electrode 30 and the drive electrode 31. As described above, a voltage stimulus 33 is applied to the drive electrode and a corresponding current is generated in the sense electrode 32 via the mutual coupling capacitance 35 and measured by a sensing circuit 36. The electrical coupling between the drive electrode 31 and sense electrode 32 may be affected by water or water vapour on the surface of the device. The guard electrode 30 and guard capacitance 34 act to reduce this coupling and, as a result, reduce the sensitivity of the capacitance sensing system to variations in humidity and condensation.
US Patent Application 2010/0079401 (Staton, Sep. 26, 2008) describes a mutual capacitance sensing device with an additional reference electrode to measure the noise injected by the touching object. As shown in FIG. 4, the device incorporates a drive electrode 40, a sense electrode 41 and the reference electrode 42. The electrodes are designed so that there is a large mutual coupling capacitance between the drive electrode 40 and the sense electrode 41 but a small mutual capacitive coupling between the drive electrode 40 and the reference electrode 42. Further, sense electrode 41 and reference electrode 42 are designed such that the capacitance between the sense electrode 41 and a touching object is the same as that between the reference electrode 42 and a touching object. Accordingly, the signal generated on the sense electrode 41 upon application of a voltage stimulus to the drive electrode 40 is larger than that generated on reference electrode 42 but the same amount of noise is injected by the touching object onto both the sense electrode 41 and reference electrode 42. The signal generated on the reference electrode 42 may therefore be subtracted from the signal generated on the sense electrode 41 to provide a measurement free from the noise injected by the touching object. However, the addition of the reference electrode into the touch panel reduces the spatial resolution of the touch panel and limits both the accuracy at which the location of the touching object may be calculated and the size of the touching object that may be detected.
Although the aforementioned methods reduce the effect of noise from selected sources, there are no known solutions to simultaneously eliminate the effects of noise from all significant sources. Also, the methods described above require the addition of electrodes into the touch panel device. This results in the added disadvantage that the spatial resolution, or accuracy at which the location of objects touching the touch panel surface can be determined, is reduced. A method of improving the immunity of a touch panel device to all significant noise sources without reducing the spatial resolution is therefore sought.